Titanium (Ti) and titanium nitride (TiN) are refractory materials with ionic structure, covalent bonding and metallic conductivity. These characteristics lead to high specific strengths at elevated temperatures, excellent mechanical, chemical and thermal stabilities, and good resistance to corrosion. These properties have made titanium and titanium nitride important building blocks in the manufacture of very large scale integrated (VLSI) circuitry, where they function as, for example, adhesion layers and diffusion barriers. VLSI fabrication also makes use of Ti--TiN bilayers on silicon substrates, where titanium functions as a getter for oxygen at the silicon interface. Such a bilayer provides significantly lower and more stable contact resistance than a titanium nitride single layer, and improved adhesion and diffusion barrier properties, compared to a titanium metal single layer, for the subsequent aluminum- or copper-based plug or interconnect layer.
The advent of ultra-large scale integration (ULSI) multilevel metallization (MLM) schemes (see, e.g., M. Rutten et al., in Advanced Metallization for ULSI Applications, ed. V. Rana et al., Mat'l Res. Soc. Pittsburgh, Pa., p. 227, 1992), has seen the development of substrates having features, such as holes, vias and trenches, of diameter less than 1 micron, often less than 0.5 micron and even less than 0.25 micron. These finely patterned substrates that are typically used in ULSI circuitry, will be referred to herein as sub-micron substrates. The sub-micron substrates used in ULSI circuitry have features with aspect ratios, i.e., the ratio of the depth to the width of a feature when viewed in cross-section, of about 3:1, sometimes 4:1 and sometimes even 6:1.
Reliable methodology has not heretofore existed for the coating of conformal, high-quality Ti and TiN films onto the finely patterned substrates used in ULSI circuitry. And yet there is a critical need for appropriate adhesion layers and diffusion barriers which may be met by Ti and TiN films. Physical vapor deposition methods, such as sputtering, which were successfully used in manufacturing VLSI devices, are unable to meet the requirements of the new ULSI devices. As feature sizes are reduced into the half-micron range and below, sputtering techniques provide undesirably non-conformal coverage. For example, sputtering causes thinning at vias, hole edges and walls, and keyholes in the vias and trenches. Further, the deposits provided by sputtering techniques frequently contain trapped sputter gas and possess a columnar growth structure which seriously inhibits their usefulness as diffusion barriers. See, e.g., S. Saitoh et al., ibid, p. 495; M. Jiminez et al., J. Vac. Sci. Tech. B9, p. 1492, 1991; and A. Noya et al., Jpn. J. Appl. Phys., 30, p. L1760, 1991. Efforts to resolve these problems through the development of modified physical vapor desorption techniques, such as collimated reactive sputtering, have been unsuccessful to date because of, for example, reduced throughput due to the use of a collimator, undesirable particulate generation, and increased sensitivity to processing conditions.
Chemical vapor deposition (CVD) is a process whereby a solid film is synthesized from the reaction products of gaseous phase precursors. The energy necessary to activate the precursors and thereby start the chemical reactions which lead to film formation, may be thermal and/or electrical, and may be reduced by catalytic activity at the surface of the substrate to be coated. It is this reactive process which distinguishes CVD from physical deposition processes, such as sputtering or evaporation. CVD potentially offers many intrinsically attractive features for fabrication of Ti and TiN films as demanded by modern microelectronics. For example, CVD can generally provide a high growth rate and conformal coating of substrates having a complex topography of trenches and vias. In addition, catalysis interaction of the substrate with CVD source precursors can possibly lead to selective metal growth.
However, as discussed below, recognized CVD methodology fails to provide Ti and TiN coatings with conformal coverage for substrates having sub-micron features as typically found in ULSI circuitry. In addition, standard CVD methodology requires processing temperatures in excess of about 650.degree. C., which is higher than can typically be tolerated in ULSI fabrication when aluminum serves as the material to provide the contacts for the circuit. The use of aluminum contacts effectively requires CVD temperatures of less than about 500.degree. C.
It is known to prepare titanium metal films by use of plasma-assisted CVD (PACVD) of TiCl.sub.4 in a mixture of nitrogen and hydrogen; by electron cyclotron resonance (ECR) plasma CVD of TiCl.sub.4 in a nitrogen atmosphere; and by atmospheric pressure CVD (APCVD) using TiCl.sub.4 and isopropylamine as coreactants. See, e.g., M. Hilton et al., Thin Solid Films, 139, p. 247, 1986, and T. Akahori et al., Proc. Int'l Conf. on Solid State Devices and Materials, Yokohama, Japan, p. 180, 1991. These efforts led to an appreciable reduction in process temperature, to within the desired range of about 350.degree. C.-500.degree. C. However, film step coverage was only 30%-70% for features of low aspect ratio, and the films exhibited undesirably high resistivities of nearly 200 .mu..OMEGA.cm. In addition, the films suffered from chlorine contamination to the extent of several atomic percent.
Early attempts at preparing titanium nitride films using CVD mostly involved coreacting titanium tetrachloride (TiCl.sub.4) and ammonia (NH.sub.3) to yield TiN films with resistivities in the range of 50 to 100 .mu..OMEGA.cm. These early attempts provided films having good step coverage and diffusion barrier properties. See, e.g., A. Sherman, J. Electrochem. Soc., 137, p. 1892, 1990. In addition, films produced thereby had impurities, mainly chlorine, at a concentration of less than about one atomic percent. See, e.g., J. Hiollman et al. in Advanced Metallization for ULSI Applications, ed. V. Rana et al., Mat'l Res. Soc. Pittsburgh, Pa., p. 319, 1992. However, the high processing temperatures involved in producing these films, typically in excess of 650.degree. C., prohibit this technology from being used to prepare ULSI devices, which can tolerate temperatures not greater than about 500.degree. C.
There are several reports of the use of organometallic precursors to prepare titanium and titanium nitride films by CVD. For example, there are several recent reports on metal-organic CVD (MOCVD) of TiN from dialkylamino derivatives of titanium of the type Ti(NR.sub.2).sub.4, where R is a methyl or ethyl group. See, e.g., R. Fix et al., MRS Symp. Proc., 168, p. 357, 1990; and K. Ishihara et al., Jpn. J. Appl. Phys., 29, p. 2103, 1990. Additional MOCVD studies involving the use of single source titanium precursors of the type TiCl.sub.2 (NHR.sub.2) (NH.sub.2 R) and TiCl.sub.4 (NR.sub.3).sub.2 have been reported. See, e.g., C. Winter et al. in Chemical Perspectives of Microelectronic Materials III. ed. C. Abernathy et al., MRS, Pittsburgh, Pa. 1992; and K. Ikeda et al., Proceedings of the 1992 Dry Process Symposium, p. 169, 1992 (using cyclopentadienyl titanium compounds, such as bis(cyclopentadienyl) titanium diazide). The use of diimine analogs of .beta.-diketonates such as Ti(NH).sub.2 C.sub.2 CHR.sub.2).sub.2 in MOCVD has also been reported. See A. Weber, The Proceedings of the Schumacher Conference (San Diego, Calif., 1993). However, the TiN films produced by MOCVD exhibit relatively high resistivities of greater than 200 .mu..OMEGA.cm, and a step coverage below 70% even for features of low aspect ratio. In addition, the films contained hydrogen concentrations of up to 50 atomic percent, and a carbon concentration of several atomic percent. These impurities are highly detrimental to the performance of the resulting films and effectively prohibit their use in ULSI devices.
MOCVD has also been studied for the preparation of titanium films. See, e.g., T. Groshens et al., in Chemical Perspectives of Microelectronic Materials III ed. C. Abernathy et al., MRS, Pittsburgh, Pa. 1992, for using MOCVD techniques with neopentyltitanium (Me.sub.3 CCH.sub.2).sub.4 Ti and silaneopentyltitanium (Me.sub.3 SiCH.sub.2).sub.4 Ti. Cyclopentadienyl-based compounds have also been explored as precursors to titanium films. See, e.g., N. Awaya et al., Japanese Patent No. 01/290,771, 1989. However, as in the case of TiN, the resulting Ti films exhibited high resistivity, and carbon and hydrogen content in excess of 10 atomic percent, making them undesirable for use in ULSI circuitry fabrication.
It is known that titanium halides will decompose to Ti at temperatures in excess of 1300.degree. C. This reaction, which is known as the Van Arkel process (see, A. Van Arkle et al. Z. Anorg. Allgem. Chem., 148, p. 345, 1925) occurs at such high temperatures that it is not useful for ULSI fabrication.
There thus exists a need for technology to provide Ti and TiN films suitable for ULSI fabrication. Such films must be of ultra-high quality, in terms of purity, with impurity concentrations well below 1 atomic percent. Also, the films should desirably exhibit a non-columnar structure in order to perform appropriately as a barrier layer. Further, the films should be conformal to the complex topography of ULSI circuitry, and provide step coverage in excess of 70%. It is desirable that technology be developed which can readily prepare single films containing either Ti or TiN, as well as bilayer films of Ti and TiN, and that such technology be amenable to process temperatures below about 500.degree. C. in order to prevent thermally induced device damage during processing.
It is especially desirable that a process be developed which allows for the preparation of the above-mentioned films sequentially and in-situ, i.e., without the necessity of transferring a substrate coated with a single film (Ti or TiN) to another reaction chamber to deposit the other film. Thus, according to current technology, the production of a bilayer typically involves the laying down of a first layer in a first reaction chamber, and then transferring the substrate to a second reaction chamber where a second layer is coated onto the first layer. Current technology does not provide a single reaction chamber with the versatility to deposit both Ti and TiN films merely by controlling the operating parameters of the chamber. As is known in the art, a process for the in-situ deposition of sequential bilayers of Ti and TiN is desirable in part because of the high affinity of titanium for oxygen and water. This affinity leads typically to contamination of the Ti film surface during transfer to a second reaction chamber where it is coated with TiN.